A heat-sensitive recording material using a color reaction of a colorless or light-colored electron-donating dye precursor with an electron-accepting compound, i.e., a two-component system heat-sensitive recording material, is disclosed, for example, in Japanese Patent Publication Nos. 14039/70 and 4160/68.
A two-component system color heat-sensitive recording material comprises a mixture of a particulate dispersion of a colorless or light-colored electron-donating dye precursor (hereinafter referred to as "color former") with a binder and a mixture of a particulate dispersion of an electron-accepting compound (hereinafer referred to as "color developer") with a binder, both heat-reactive components being coated on a support with a binder interposed therebetween. In such an arrangement, when one or both of the two heat-reactive components are melted on heating, the color forming compounds are brought into contact with each other to undergo a color reaction by which recording is effected. Such a two-component system color heat-sensitive recording material is advantageous in that it undergoes a primary color reaction which requires no development, that the paper quality can be nearly that of commonly used paper, that it can be easily handled, that it provides a high color density, and that it provides various color hue heat-sensitive recording materials. Thus, the two-component system color heat-sensitive recording material finds the widest application as a heat-sensitive recording material.
In particular, the heat-sensitive recording material has recently found increasing application in facsimile telegraph, recorder, and printer devices. In order to increase the recording speed of, for example, a facsimile device, the rate of travel of the recording head is increased and the duration of the pulse of thermal energy is shortened. This has required that heat-sensitive recording material be developed to be sensitive enough to a short pulse width, i.e., small heat energy to develop color with a sufficient density. Various attempts have been made as disclosed in Japanese Patent Application (OPI) Nos. 18739/83, 67083/84 and 54884/85 (the term "OPI" as used herein means an "published unexamined Japanese patent application").
A problem in such high speed heat-sensitive recording is that sufficient heat conduction cannot be made between the thermal head and the heat-sensitive recording paper with which the thermal head is in contact, making it impossible to provide sufficient recording density. This has been a major factor that prevents the heat-sensitive recording paper from being used in high speed recording. A thermal head is an assembly of dot-shaped electrical resistance heating elements which respond to a recording signal to produce heat. When these electrical resistance heating elements are brought into contact with the heat-sensitive color forming layer, the heat-sensitive color forming layer is molten to develop color. In order to provide sharp and high density recording, an excellent dot reproducibility is required. In other words, the thermal head and the heat-sensitive color forming layer need to be in close contact with each other to provide a more efficient heat conduction therebetween so that dots having a shape corresponding to that of the dot-shaped heating elements can be formed on the heat-sensitive color forming layer in fully developed colors in full response to high speed recording signals. At present, however, only a small percentage of heat produced by the thermal head is transmitted to the heat-sensitive color forming layer. Thus, the efficiency of heat conductance between the two components is still extremely low.
Heretofore, several approaches have been proposed to improve the surface smoothness of the heat-sensitive color forming layer so that the thermal head and the heat-sensitive color forming layer come into as close contact as possible with each other.
For example, Japanese Patent Publication No. 20142/77 describes a method which comprises treatment of the surface of a heat-sensitive color forming layer to a Bekk smoothness of 200 to 1,000 seconds. Japanese Patent Application (OPI) No. 115255/79 describes that when a Bekk smoothness is from 200 to 1,000 seconds, the heat-sensitive color forming layer can respond only to heat impulses of from about 5 to 6 milliseconds, and that for high-speed recording using heat impulses of not more than 1 millisecond it is necessary for the surface of the heat-sensitive color forming layer to be made smooth to such an extent that the Bekk smoothness is not less than 1,100 seconds. However, when the Bekk smoothness is increased to not less than 1,100 seconds, color fog is produced upon application of pressure. The formation of color fog is prevented by using a base paper which has been previously made smooth to an extent that the Bekk smoothness is 500 seconds or more. Japanese Patent Publication No. 43318/85 describes a method which comprises the use of a extracted hot water-containing base paper having a pH value of 6 to 9 to prevent fog. Japanese Patent Application (OPI) No. 56086/78 describes that the surface roughness, Ra, of the heat-sensitive color-forming layer is made to be less than 1.2 .mu.m, and the glossiness less than 25%.
In all the above-described prior art techniques, the smoothness of the heat-sensitive color-forming layer is increased only by calender processings such as super calendering, machine calendering, and gloss calendering. This calendering is applied to the base paper alone, or the base paper and the heat-sensitive paper, or the heat-sensitive paper alone. In the heat-sensitive recording paper, however, as the smoothness is increased by the calendering in order to increase the recording density, adherence and accumulation are increased. In practical use, therefore, the smoothness is suppressed to a suitable level so that the recording density and the occurrence of adherence and accumulation are properly balanced. In the prior art techniques, regardless of the smoothness level, the resulting heat-sensitive recording paper is unsuitable for practical use for high-speed recording in respect of recording density and recording stability.
The term "adherence" (sticking) as used herein refers to a phenomenon wherein the thermal head adheres to the heat-sensitive color-forming layer, thereby producing stripping noise and lowering the dot reproducibility. The term "accumulation" (piling) refers to a phenomenon wherein heat-melted products of the heat-sensitive color-forming layer accumulate on the thermal head, thereby lowering the recording density and dot reproducibility. Both of these phenomena inhibit stable recording.
Another disadvantage as encountered in applying the calender processing to the heat-sensitive recording paper is that color fog is formed by pressure, resulting in an increase in the density of the background of the recording paper. Similarly, in the calendering processing of the base paper, so-called cockle, wrinkles, etc. due to unevenness in basis weight develop. Thus it is limited in its practical use. As described above, the attempt to increase the smoothness of the heat-sensitive color-forming layer by calendering processing so as to increase the recording density has met with only limited success, and the resulting heat-sensitive recording paper is not sufficiently satisfactory for use in high-speed recording.
Japanese Patent Application (OPI) Nos. 2397/85 and 176091/84 describe a method which comprises incorporation of an anionic copolymer. However, this method leaves to be desired in that a sufficient recording density cannot be obtained.
Japanese Patent Application (OPI) No. 184875/85 describes a method which comprises incorporation of a maleic anhydride copolymer/polyalkylene polyamine reaction product as a cationic softening agent. However, this method is disadvantageous in that a base paper thus prepared has a poor surface strength, causing dusting or fluffing upon calendering.
Such a manufacturing trouble occurs particularly when the base paper is neutralized and comprises calcium carbonate as a filler material.